This Invention relates to a technique for compensating for the inhomogeneity of the field generated by the RF coil in a nuclear magnetic resonance experiment.
This work addresses a problem which occurs in Nuclear Magnetic Resonance (NMR) experiments. These experiments include NMR spectroscopy (MRS), and NMR imaging (MRI). NMR experiments may be performed on both living subjects and samples such as chemical solutions. Some examples of NMR experiments are diagnostic medical imaging (MRI), and determination of chemical structures by NMR spectroscopy.
A central feature of all NMR experiments is the precise spatial and temporal control of a number of magnetic fields applied to the sample or subject (for the case of in vivo experiments).
`B.sub.0 ` refers to the uniform, strong static magnetic field, usually produced using a superconducting magnet. This field is fine-tuned using "shim" coils. NMR imaging and localized spectroscopy additionally use switched linear B.sub.0 field gradients. Some forms of spectroscopy also employ switched gradients. B.sub.Z is the component of magnetic field in the z-direction. The z-direction is defined by the direction of the B.sub.0 field.
`B.sub.1 ` refers to the radio frequency (RF) field, which is normally pulsed, and is produced using a tuned probe (the RF coil). The most basic function of the transmitted RF pulse is to excite the spins (magnetic atomic nuclei) away from their equilibrium alignment. This causes a small NMR RF signal to be emitted by the sample which is detected using an RF receiver coil and sensitive receiver electronics. The same RF coil is often used for both transmission of RF pulses and reception of NMR signals from the sample.
RF pulses are an essential part of any Fourier transform (FT) NMR experiment, and the use of pulses of the correct amplitude is usually most important. It is therefore desirable to use a coil which has a uniform B.sub.1 field over the sample, in order that the effect of the RF pulse is the same throughout the sample. However, compromises are often made in order to increase signal-to-noise ratio (SNR) and additionally various technical problems exist in the production of a uniform field in certain circumstances.
The B.sub.1 field is in the radio frequency (RF) range for NMR. Some Electron Spin Resonance (ESR), also known as Electron Paramagnetic Resonance, experiments use very similar methods to NMR, therefore the methods discussed could also in principle be applied to ESR, but B.sub.1 is normally in the microwave region for ESR experiments. For simplicity the term "RF" will be used throughout this document.
Incorrect B.sub.1 field intensity can occur for a variety of reasons, these include incorrect calibration of the RF pulse amplitude, instability or drift of the RF amplifier or other RF electronics, non-uniform RF transmitter coils and by interactions with lossy samples. The result is that improper flip angles are produced, resulting in a wide variety of undesirable effects in the NMR experiments. The SNR may be reduced, and the measurement accuracy is impaired.
In one solution, a uniform transmitter coil and a separate local receiver coil (surface coil) is used. This commonly used arrangement can be suitable when the region of the sample being studied is a relatively small fraction of the total sample size, and is located near to the surface. The large transmitter coil has effectively a uniform field over the volume of interest eliminating the problem, while the small receiver coil allows good sensitivity for reception of the NMR signal.
One limitation of this approach is that there is not always enough space for two RF coils to be used. Clinical head imaging is an example where it is preferable to use only one RF coil, and so coil homogeneity may be sacrificed for sensitivity. A large RF coil requires a more powerful RF amplifier and results in RF heating of a larger part of the body. Furthermore, the more space that is required for RF coils, the less space is available for the sample, and for the gradient and shim coils. The use of larger gradient coils requires higher currents which are more expensive to produce. If a larger magnet is required to accommodate larger gradient and RF coils then this is also more expensive.
In another solution a special RF coil can be designed to compensate for RF losses and interactions with the sample. In some cases an RF coil which has a uniform field in air, nevertheless produces a non-uniform field within a sample. It is sometimes possible to design an RF coil, albeit with decreased efficiency, to compensate for this. Such an RF coil would have to be designed for a specific sample conductivity distribution.
Another widely used technique uses special pulse shapes particularly modification of the amplitude and phase of the RF pulse waveforms. By using specially designed RF pulses the effects of variations of the RF field strength can be reduced. A unique advantage of this approach is that errors in setting the gain of the RF transmitter are also compensated. The two main categories of compensated RF pulses are:
Composite RF pulses (1) PA1 "Adiabatic" pulses (2), (3). PA1 providing at least one magnet generating a magnetic field B.sub.0 which is substantially static during the experiment and substantially spatially uniform over the sample, the magnetic field direction defining a z-reference axis along the longitudinal direction of the magnet; PA1 providing at least one radiofrequency (RF) coil and applying an RF signal (normally in the form of a series of one or more pulses) to the RF coil to generate an RF field B.sub.1, the RF coil and sample having characteristics such that the field generated is non-homogeneous (spatially non-uniform) over at least part of the sample; PA1 and compensating for the non-homogeneity of the RF field by applying an additional phase (or an equivalent frequency modulation) to an initial RF pulse, PA1 consisting of a time-dependent phase function, .beta.(t). The phase modulation of an initial pulse B.sub.1.sup.init by the time-dependent phase function, .beta.(t), is represented by: EQU B.sub.1M =B.sub.1.sup.init exp(-i.beta.(t)). PA1 a) purpose: B.sub.1 insensitive RF pulses for NMR imaging and spectroscopy PA1 b) method: modulation of RF pulse waveforms using an oscillatory frequency modulation function PA1 c) a moderate degree of immunity to RF inhomogeneity is imparted to the RF pulse PA1 d) effective with a wide range of existing RF pulses, retaining the frequency response over a specified range of frequency offsets and retaining other desirable features. These pulses are therefore much more flexible than adiabatic pulses. PA1 e) The main advantage of these pulses is that, over a specified bandwidth, the frequency response of an existing RF pulse may be retained, but the pulse becomes, to an approximately first order, immune to B.sub.1 variations. Therefore excellent slice profiles and other desirable properties (e.g. spin refocusing) becomes available. PA1 f) the B.sub.1 pulses can be used on existing MRI equipment. PA1 g) RF power requirement may be higher than for conventional pulses, PA1 h) Aliasing of response at the modulation frequency and above, leading to a limitation on the usable frequency bandwidth. PA1 i) Suitable for use with surface coils or any other coils with non-uniform B.sub.1 response including high field volume coils. PA1 j) Reduced immunity to system and subject variations, less accurate adjustments or RF amplitude needed. PA1 k) NMR spectroscopy: the aliasing of response does not matter if it is outside the spectral bandwidth of interest, also the higher RF power requirements are less of a restriction than they are in vivo. These pulses could make many high resolution NMR experiments much less sensitive to experimental variations. PA1 l) Some of these pulses are slice selective, and may be used for multislice imaging, with some restrictions, and so could become routinely used in some MRI applications. PA1 a) Some given complex B.sub.1 pulse, B.sub.1.sup.init, ( hereafter referred to as the "initial pulse"), which may be of any form, subject to the condition that the initial pulse has one or more identifiable desirable properties, for instance, excitation or refocusing properties, or a specific frequency response (e.g. slice selection for MRI). Many such pulses are prior art. The initial pulse may be of a very simple form e.g. a rectangular pulse (hard pulse) or a sinc pulse (sin(x)/x), and is not necessarily phase modulated. PA1 b) A phase (or equivalently frequency) modulation of the initial B.sub.1 pulse, B.sub.1.sup.init, PA1 c) Phase modulation of B.sub.1.sup.init a time-varying phase function, .beta.(t) , may be represented by: B.sub.1M =B.sub.1.sup.init exp (-i.beta.(t)). Angular frequency is the rate of change of phase so this phase modulation may also be expressed as a frequency modulation of .OMEGA.(t)=d.beta.(t)/dt. The form of frequency modulation function claimed is one that is substantially oscillating, i.e. reversing in sign multiple times during the pulse, for example: .OMEGA.(t)=2.pi.Csin(.PHI.+.omega.t) radians. Furthermore this frequency oscillation must be rapid relative to the amplitude modulation of the initial pulse, B.sub.1.sup.init. For example if B.sub.1.sup.init varies in amplitude predominantly over a time scale of tp, i.e. operating over a frequency bandwidth of approximately 2.pi./tp radians, then the condition is that: .omega.&gt;2.pi./tp radians, where tp is the pulse duration. PA1 d) The modulation can be physically achieved by means of: PA1 phase or frequency modulation of the B.sub.1 envelope of the initial B.sub.1 pulse, or PA1 frequency modulation of the B.sub.1 pulse transmitter, or PA1 field modulation of the B.sub.Z field. PA1 e) the identified desirable properties of the initial B.sub.1 pulse are substantially preserved over a "usable frequency range", when an appropriate (higher) B.sub.1 intensity is used in place of the intensity of the original RF pulse. "Usable frequency range" is defined as a range of frequencies over which the desired properties hold. For the case of slice selection this includes both the slice bandwidth and the undisturbed region outside the slice. PA1 f) by the rapidity of the frequency modulation relative to the amplitude modulation of the pulse. PA1 g) The difference in behavior: adiabatic sweep pulses can have large immunity to B.sub.1 intensity over a wide range of B.sub.Z values, but at the expense of the loss of other desirable pulse properties, (e.g. the control of phase response required for excitation or refocusing). PA1 h) adiabatic pulses cannot be decomposed into the two components of a rapid frequency modulation and an initial pulse with the same properties as the modulated pulse. PA1 providing at least one magnet generating a magnetic field B.sub.0 which is substantially static during the experiment and substantially spatially uniform over the sample volume, the magnetic field direction defining a z-reference axis along the longitudinal direction of the magnet; PA1 providing at least one B.sub.Z field coil (`gradient coil`) and applying a signal to the coil resulting in a B.sub.Z magnetic field which in addition to the B.sub.0 field is applied to form a variable magnetic field in which the sample is located, the B.sub.Z field coil producing a B.sub.Z field component generating a B.sub.Z magnetic field (a magnetic field in the z-direction) which is spatially varying over at least part of the sample; PA1 providing at least one radio frequency (RF) coil and applying an RF signal (normally in the form of pulses) to the RF coil to generate an RF field B.sub.1, the RF coil and sample having characteristics such that the resultant RF field generated is non-homogeneous (spatially non-uniform) over at least part of the sample; PA1 and compensating at least partially for the effects of the non-homogeneity of the RF B.sub.1 field by applying a signal to the B.sub.Z gradient coil and producing a B.sub.Z field, during the application of the RF B.sub.1 field. PA1 a) Uses additional B.sub.Z gradient fields, to spatially modify the response of RF pulses; PA1 b) The gradient coil field profile Is designed to be correlated with the B.sub.1 field profile. This allows the frequency shifts, due to the B.sub.Z waveform, to compensate for the effects of the spatial variation in the B.sub.1 amplitude. PA1 c) Simultaneous B.sub.1 and B.sub.Z pulses are used to produce `Correlated Rotations` (COR). This provides greater control over the spin system than using B.sub.1 pulses alone. PA1 d) The general ideas expressed here can be used to design specific instances (specific pulses). Building on these ideas, other specific pulses can be designed, which may be superior to the examples given here. PA1 e) COR pulses can be used on existing NMR equipment, however there are a number of hardware and software modifications which are suggested by this work. PA1 f) COR pulses allow the theoretical absolute minimum RF power to be used, because the compensation is performed by the gradient coil, rather than by the B.sub.1 waveform, this introduces the possibility that, for some COR pulses, the pulse duration and pulse voltages can be the absolute minimum. Some of the pulses shown hereinafter use a square RF pulse, which is the most efficient pulse for on-resonance spins. PA1 g) The COR pulse method also simplifies RF pulse sequences, and shortens the pulse sequence. PA1 h) Comparison with adiabatic pulses; adiabatic pulses, and some composite pulses, vary the frequency globally (i.e. over the whole sample), there being no correlation between B.sub.1 ad B.sub.Z fields. Pulses which vary the frequency as a (fixed) function of position we will denote as `COR1 pulses`, and those which vary the frequency as a time-dependent function of position: `CORM pulses`. Note that COR pulses do not necessarily rely on frequency sweeps, although some COR pulses do have this feature.
General disadvantages are that these pulses are either longer, or require more RF power than uncompensated pulses or both. Additional RF power entails either higher RF voltage or a longer pulse (or both). For In vivo experiments especially, the RF power deposition is limited to a safe maximum value, which varies depending upon specific experimental conditions. In clinical imaging and especially when operating at higher B.sub.0 field values considerations of safety and sample heating due to deposition of RF energy imposes significant restrictions on the execution of NMR measurements. Any methods requiring less RF power therefore would have an important advantage over competing methods.
Specific disadvantages of composite pulses are that they generally are compensated for either off-resonance performance, or for B.sub.1 intensity variations, but not both. Furthermore composite pulses, built-up from a series of short, "hard" pulses are not suitable for slice selection in MRI.
Specific disadvantages of adiabatic pulses are that slice selective excitation and slice selective refocusing cannot be satisfactorily achieved, with the result that these pulses are not routinely used in MRI. Good adiabatic selective inversion pulses do exist.
Other relevant prior art, which provides further detail of the above, is shown in:
1. P. A. Bottomley et al BIRP, An improved implementation of low angle adiabatic excitation pulses, Journal of Magnetic Resonance Series A 103, 242 to 244 (1993).
2. J. P. Hornak et al, Magnetic field mapping, Magnetic Resonance Medical 6 158 to 163 (1988).
3. M. H. Levitt Prog. NMR Spectroscopy 18:61 to 122 (1968).
4. M. Garwood et al, B1 Insensitive adiabatic pulses, NMR Basic Principles and Progress Vol 26 Springer-Verlag, Berlin Heidelberg (1992).